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The engine's ignition distributor

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Before the introduction of the electric starter motor, most vehicles used a hand crank and a magneto for ignition. Later versions teamed a magneto, hand crank and electric starter for improved reliability: Even if the car's crude early storage battery failed, the vehicle could still be started, though it would take more effort. Once the storage battery became standard, automakers were able to explore new ignition concepts. No longer was the ignition control device required to produce its own electrical output, and the modern breaker point distributor--or, as some called it, timer--was born.
Electric ignition was, interestingly, the first form of combustion engine ignition. Frenchman Étienne Lenoir used high-tension electricity in 1860 to ignite the combustible fuel in a gasoline engine; he was credited with the first spark ignition internal combustion engine, which he put to stationary use. Unfortunately, electrical equipment was then in a primitive state, and it did not prove reliable for an ignition system. Various other methods were tried, notably the hot-tube system, which utilized a red-hot platinum tube to initiate combustion in the cylinder.
A major problem facing automotive engineers was that the ignition systems under development in that era may have worked for a stationary engine, but couldn't be employed on a vehicle that was intended to go anywhere the driver wanted. Another concern was the need to properly time the ignition events for the speed and load of the engine. Stationary applications did not have to worry too much about this, since the powerplant would usually only operate at a single RPM rate.
For many years, two ignition systems were used for engines. These were the self-contained high-tension magneto and the battery and coil system. The magneto was pioneered by Robert Bosch in 1901, and led to his forming the company that still bears his surname. The outbreak of World War I resulted in further development of the magneto in England by E. A. Watson, as well as ignition pioneers L. Griffiths, H.W.F. Ireland and J.D. Morgan.
The battery and coil system, which was considered the best modern ignition, was developed in 1912 by well-known American engineer Charles F. Kettering; he also introduced the electric self-starter, which quickly became known as the starter motor. Kettering eventually became president of the world-renowned General Motors Research Laboratory.
Kettering's battery and coil/distributor ignition system was eventually adopted by gasoline engine makers the world over. It was a natural choice for automakers because the intensity of the spark was consistent regardless of engine speed, allowing for easier starting during crankover. This system also had very few moving parts, improving reliability. And with no heavy components other than the storage battery, which by then was considered a standard piece of automotive equipment, the distributor ignition carried no weight penalty and reduced the mass of the engine.
The Distributor
As its name implies, the ignition distributor is responsible for parceling out electricity to the proper spark plug and at the correct time. This, however, isn't its only job--it also needs to control the ignition coil by switching the feed voltage on and off. Initially, this was accomplished by means of breaker points; in the 1970s, breaker points were replaced with a more efficient electronic transistor device.
The distributor consists of a metal housing with a shaft that usually connects to the engine's camshaft. The distributor's shaft rides in a bronze bushing in the case; some applications employ a roller or ball bearing to reduce friction and improve wear characteristics. Traditionally, the shaft has centrifugal weights that are RPM-sensitive and controlled through spring tension. As the engine's RPM increases, the distributor shaft also revs up, and the centrifugal force pulls the weights away from their at-rest position. The weights then act upon a mechanism that advances the timing by altering the trigger point of the breakers, which are operated via a cam that is also part of the shaft. The gear-driven shaft is also a mooring for the rotor that, when in alignment with the proper terminal of the distributor cap, sends the high-voltage output of the ignition coil to the appropriate spark plug via the spark plug wire. In order to properly discuss all the numerous duties of the distributor, let's break it down into parts.
The distributor housing is nothing more than a metal case in which to store the components; it also provides an attachment for the distributor cap. The cap is the connection point for the ignition, or high-tension wires, that go to the spark plugs. The cap was originally made from Bakelite resin, but was eventually made from a plastic or phenolic material.
The distributor cap has metal tracks inside; when the rotor aligns with these tracks, it sends electrical current to the proper spark plugs. There is a gap between the rotor tip and the metal contacts. Electricity must jump this rotor gap to begin its journey to the cylinder. Since the distributor cap operates under atmospheric pressure, the amount of energy required to bridge the gap is minimal when compared to the arcing of the spark plug under compression pressure. Depending on the system's design, the distributor cap can either house the ignition coil or allow connection from a remotely located coil through a high-tension wire.
The earliest ignition distributors employed only a centrifugal advance system. Engines from the 1950s and later also incorporated a vacuum advance canister which, instead of working on the shaft, moved the breaker plate that held the points (trigger mechanisms). Under light load, which equates to high engine vacuum conditions, the breaker plate advanced against the rotation of the distributor cam (not the engine camshaft) and allowed the ignition event to begin earlier in the piston's travel past top dead center. This worked as an adjunct to the centrifugal advance system and was meant to improve engine fuel efficiency.
The distributor needs to allow changes in ignition timing because of the varying rate at which the spark plug's flame expands in the cylinder bore. At low levels of cylinder fill, which is described by volumetric efficiency, the lack of internal charge motion from the partially filled cylinder causes the mixture to burn slower. Thus, the arcing of the spark plug has to begin sooner, so that the cylinder pressure rise through the expansion of the flame keeps up with the velocity of the piston. If this didn't happen, the ignition event would occur too late, and most of the energy from the fuel would be wasted, since it would not work against the piston but rather go out the exhaust valve.
The electrical portion of the distributor has both a primary (low voltage) side and secondary (high voltage) circuit. The primary circuit consists of the breaker points and a wire that connects them to the switched side of the ignition coil. There is also a capacitor, often called a condenser. The condenser, too, is attached to the distributor, and can be located either under the distributor cap or be mounted on its exterior.
The condenser is meant to absorb the excess electrical energy at the breaker points as the mechanical cam forces the points apart. If the condenser was not there, then the points would arc as they opened, not allowing the field in the ignition coil to collapse, since it would still be supplied with electron flow. The spark plug fires just as the breaker points open due to the field in the coil no longer being supported by the low voltage from the battery. It then collapses across the additional windings that multiply the output. This becomes the secondary or high-tension side of the coil.
Though the distributor has no influence other than controlling the "on" and "off" states of the coil through the breaker points, its job is to distribute the high-tension voltage to the spark plugs through the intermediate components of the rotor and distributor cap.
The helical cut gear that connects the bottom of the distributor shaft to the camshaft needs to be timed correctly to coincide with the opening and closing of the breaker points via the distributor cam, along with the clock position of the housing so that the proper spark plug receives the secondary voltage. In many engines, the distributor is also responsible for running the oil pump through another shaft that connects at the bottom of the gear. The engine's camshaft turns the distributor, and as the distributor's shaft rotates, the oil pump is operated. Thus, there is a free play specification machined into the components so that the oil pump does not bind up the distributor--particularly when the engine is cold and the oil's resistance to being pumped is much higher.
Though the basic appearance of the distributor varied by manufacturer, all of the components were pretty much the same--they were just packaged in a unique way. The biggest change to the distributor came in 1962. General Motors offered as an option on some Pontiac models a breakerless ignition system that had electronic components in place of the breaker points. This resulted in a more powerful ignition, since the transistors were able to withstand a higher amount of primary force than the breaker points could and still produce a sufficient service life. But the car-buying public did not immediately embrace this newfangled design.
Eventually, federal emission standards forced the public to accept electronic ignition housed in a standard distributor. In 1973, Chrysler converted its distributor to a breakerless design; the domestic industry followed for the 1975 model year with the advent of the catalytic converter. This was the first significant change made to Kettering's original 1912 design.
The ignition distributor was eventually phased out, replaced by coil packs and a camshaft and crankshaft sensor. But change was not completely integrated into the market until almost 2000. Kettering's basic design had served countless engines for almost 100 years before it could be improved upon enough to be fully replaced: A true mechanical marvel if there ever was one!

This article originally appeared in the October, 2010 issue of Hemmings Classic Car.